1. Field of the Invention
The invention relates to an electrostatic discharge protection circuit of an integrated circuit and, in particular, to an electrostatic discharge protection circuit with a diode structure.
2. Description of Related Art
Using two field effect transistors (FET) as an electrostatic discharge protection circuit is common in the design of integrated circuits. The electrostatic discharge function is provided by the parasitic diode(s) inside the field effect transistors, thus the electrostatic protection circuit should have a diode structure.
Referring to FIG. 1, the device structure equivalent circuit of a twin-diode electrostatic discharge protection circuit is formed in a semiconductor substrate and a cross-sectional view of the device structure is shown in FIG. 2. Twin-diodes D1 and D2 are shown in FIG. 1 with diode D1 connected between a power source V.sub.DD and an output/input bonding pad 10 while diode D2 is connected between the output/input bonding pad 10 and a ground point. Because the diode structure displays enormous ability in protecting against electrostatic discharge and has a very low input capacitance, it is suitable for use in protecting integrated circuits from electrostatic discharge damage.
When the twin-diode electrostatic discharge structure receives an external electrostatic force, it will act differently, depending on the kind of electrostatic force applied to the structure. By using 4 kinds of human body electrostatic force models applied to the protection circuit and using the cross-sectional view of the device structure shown in FIG. 2, the manner in which the protection circuit operates and its drawbacks can be made more easily understandable.
The first model is an electrostatic force which is positive with respect to a ground. This positive electrostatic force can cause the junction of an n+ diffusion region 34 in a p-type well region 30 to breakdown, and the lateral pnp bipolar junction transistor formed by a p-type well region 30, an n-type substrate 20 and a p+ diffusion region 24 to discharge the electrostatic force.
The second model is an electrostatic force which is negative with respect to a ground. This negative electrostatic force can create a forward bias voltage between an n+ diffusion region 34 and a p+ diffusion region 32 within p-type well region 30, the forward bias voltage discharging the electrostatic force.
The third model is an electrostatic force which is positive with respect to the V.sub.DD. Since the electrostatic force is higher than the high power source V.sub.DD, a forward bias voltage is created from a p+ diffusion region 24 to all n+ diffusion regions 22, 26, and 28 in the n-type substrate 20 connected to V.sub.DD. The electrostatic force is then discharged by the forward current.
The fourth model is an electrostatic force negative with respect to the high voltage V.sub.DD. This negative electrostatic force can cause the junction of a p+ diffusion region 24 in the substrate 20 to breakdown. Moreover, a current path in a vertical npn bipolar junction transistor will be created by the n+ diffusion region 34 in p-type well region 30, the p-type well region 30 itself, and substrate 20. The current gain of the vertical bipolar junction transistor is around 20.about.110, so that once the junction of a p.sup.+ diffusion region 24 breaks down, a large electrostatic current flows from a well region 30 to substrate 20 forming a high-level injection phenomenon. Furthermore, because of the close spacing between n.sup.+ diffusion region 22 and p-type well region 30, current crowding occurs near the junction between the p-type well region 30 and the n-type substrate 20, resulting in the damage of the device structure.
During electrostatic discharge of the fourth electrostatic model type the device structure can be easily damaged, thereby affecting the function of the protection circuit. Because the current path in the vertical transistor with high current gain is not completely used, but rather causes device electrostatic damage, a new design for an electrostatic discharge protection circuit with a traditional diode structure is required for providing complete protection of the integrated circuit.